NAME

regexp - Plan 9 regular expression notation

DESCRIPTION

This manual page describes the regular expression syntax used by the
Plan 9 regular expression library regexp(3). It is the form used by
egrep(1) before egrep got complicated.
A regularexpression specifies a set of strings of characters. A
member of this set of strings is said to be matched by the regular
expression. In many applications a delimiter character, commonly
bounds a regular expression. In the following specification for
regular expressions the word ‘character’ means any character (rune) but
newline.
The syntax for a regular expression e0 is
e3: literal | charclass | ’.’ | ’^’ | ’$’ | ’(’ e0 ’)’
e2: e3
| e2 REP
REP: ’*’ | ’+’ | ’?’
e1: e2
| e1 e2
e0: e1
| e0 ’|’ e1
A literal is any non-metacharacter, or a metacharacter (one of
.*+?[]()|\^$), or the delimiter preceded by
A charclass is a nonempty string s bracketed [s] (or [^s]); it matches
any character in (or not in) s. A negated character class never
matches newline. A substring a-b, with a and b in ascending order,
stands for the inclusive range of characters between a and b. In s,
the metacharacters an initial and the regular expression delimiter must
be preceded by a other metacharacters have no special meaning and may
appear unescaped.
A matches any character.
A matches the beginning of a line; matches the end of the line.
The REP operators match zero or more (*), one or more (+), zero or one
(?), instances respectively of the preceding regular expression e2.
A concatenated regular expression, e1e2, matches a match to e1 followed
by a match to e2.
An alternative regular expression, e0|e1, matches either a match to e0
or a match to e1.
A match to any part of a regular expression extends as far as possible
without preventing a match to the remainder of the regular expression.